Cell based multilevel voltage source converters are known to be used for converting between AC and DC voltages for instance between AC and DC power transmission systems.
The cells of these converters are made up of switches, often in the form of pairs of switching elements with antiparallel diodes, where the switching elements may be Integrated Gate Bipolar Transistors (IGBTs). In order to be able to control the switching elements, the cells are often equipped with gate control units, which are responsible for providing gate voltages of sufficient magnitude to be able to operate the switching elements.
It is often of interest to only use the power of the AC or DC system for energizing these gate control units.
In this way it is possible to avoid using an auxiliary power supply system, which increases the reliability of the power supply. An auxiliary power supply system may be expensive but also difficult to provide, especially if the converter is to be located in a remote and inaccessible location.
Furthermore, if the converter is to be black started there is no accessible power supply to all of the gate units. When performing energizing only some of the gate control units will receive power, the others will not.
Black starts may be required in for instance offshore windfarm applications.
Black starts in relation to voltage source converters is described in US 2009/0309421 and in WO 2007/055593. However none of these documents discuss the black start of a cell based multilevel voltage source converter.
There is therefore a need for providing a black start of a multilevel voltage source converter.